Say you want to use a PHP include inside one of your pages, but they all have a .html extension. If you don't want to convert your page to a .php extension, just add this one simple line of code to your .htaccess file.
<table><tbody><tr><td><strong>CODE</strong></td></tr><tr><td>AddType application/x-httpd-php .php .html</td></tr></tbody></table>
This will treat all the code in your pages as PHP before it displays it on the page.